Re: Concerning projectile motion:

Well, it depends.  In the absence of any air resistance, the dropped
and fired bullets fall at exactly the same rate, because, as you point
out, the horizontal and vertical components of velocity are independent.
More accurately, we would say that the horizontal and vertical components
of the force on the bullet are independent.  If gravity is the only
force, we see that the vertical gravitational force is not affected
by the bullet's horizontal motion.

Let's now put in air resistance, and the situation get's more complicated.
In the simplest case, we take the air resistance to be directly
proportional to an object's velocity, and in the opposite direction
of the velocity.  The proportionality is going to depend on the
amount of surface area presented in the direction of motion, e.g.,
a large flat board clearly experiences more air resistance than an
arrow, if both are moving at the same velocity.  For the bullet, what
this means is that if the bullet doesn't tumble at all, remaining
horizontal in both the fired and dropped cases, then in both cases
the bullet falls at the same rate.  The reason for this is that in
the case where air resistance is directly proportional to the, the
force in the vertical direction is still independent of the horizontal
velocity.  In physics, we call such forces "separable", because they
can be separated into their various independent components which
don't depend on the other components.

The simple model of air resistance described above is pretty accurate
in describing reality, but not perfect.  Air resistance is generally
more complicated.  For instance, if we were to go one step better
in describing air resistance, we might add a term which is still in
the direction opposite to the velocity, but proportional to the *square*
of the speed.  In this case, the air resistance becomes a non-separable
force, and the dropped and fired bullets do not fall at the same rate.
However, in reality, the difference is very small.

For literature, a good starting place would be a basic physics textbook
like Halliday and Resnick.  I wasn't able to find anything that looked
relevant in our science library, however, you might check for publications
from the Army or other military branches.  If anyone has studied the
details of bullet dynamics, it would be them.